Loop heat pipe apparatus for heat transfer and thermal control

ABSTRACT

Loop heat pipe apparatus ( 1 ) for heat transfer and thermal control, using a two-phase fluid as a working media and comprising:
         at least one evaporator ( 2 ) to be connected with a heat source and comprising a thermal stabilization-compensation chamber ( 10 ) attached to the at least one evaporator ( 2 ) and a secondary capillary pump ( 40 ) located inside the thermal stabilization-compensation chamber ( 10 ),   at least one condenser ( 27 ) to be connected with a heat sink,   liquid lines ( 24 ) and vapour lines ( 28 ) connecting the at least one evaporator ( 2 ) and the at least one condenser ( 27 ), and   a remote compensation chamber ( 20 ),
 
wherein the thermal stabilization-compensation chamber ( 10 ) comprises a two-phase reservoir ( 5 ) and a liquid accumulator reservoir ( 6 ) separated by a heat exchange surface ( 15 ), such that the remote compensation chamber ( 20 ) is hydraulically connected with the two-phase reservoir ( 5 ) and the liquid accumulator reservoir ( 6 ).

FIELD OF THE INVENTION

The present invention relates to a heat transfer and thermal controldevice, in particular for use on a spacecraft, and more particularly theinvention is directed to a heat transfer and thermal control device withtwo-phase capillary driven loops.

BACKGROUND OF THE INVENTION

Most of the components and subsystems of a spacecraft must operate inrestricted temperature ranges. This makes thermal control a key matterin the design and operation of a spacecraft with a significant weight,power and cost impact in the overall spacecraft budget.

Spacecraft thermal control relies on the global spacecraft thermalbalance: the heat loads must be rejected to deep space that works as athermal sink. Since no matter links this sink and the spacecraft, thisrejection is made by thermal radiation through dedicated radiatorsinstalled on the satellite external surfaces.

Spacecraft thermal loads come from the internal spacecraft equipmentdissipation and, externally, from the sun and the earth or from thecelestial bodies around which the spacecraft orbits. The thermal systemsused in spacecrafts must therefore be able to control equipment whichoperates at a specified range of temperatures and also discontinuously.

At present, known thermal devices for controlling thermal loads inspacecraft are two phase heat transfer loops which are also known inengineering practice as capillary driven and mechanically pumped loopsor heat loops. The purpose of these devices in a spacecraft is totransfer heat between a heat source (for instance, an electronicelement) and a heat sink (typically, the space). In two phase heattransfer loops heat is transferred through an evaporation-condensationcycle of a working fluid kept inside a hermetically sealed container.Capillary driven loops have a special porous structure, called capillarypump or wick, serving for working fluid continuous circulation in thesystem. The wick is always located in the evaporator of the capillarydriven loop. The evaporator is attached to a heat source.

The above-mentioned capillary driven loop technology has found a wideapplication for thermal control systems in many spacecraft applications,that usually use loops with a single evaporator. However, manyapplications require thermal control of large thermal contact surfacepayloads or multiple remotely located heat sources.

Developers of multiple evaporators and multiple condenser designs ofcapillary driven loops (known in engineering practice as loop heat pipes(LHPs), capillary pumped loops (CPLs), hybrid two-phase heat loops)intend to create thermal control systems having the followingcharacteristics: optimized functional layout, scalability,expandability, effective heat loads sharing, flexibility in componentslocations, thermal coupling between separate radiators and minimizedmass and volume.

The LHP technology was initially invented in the Soviet Union, and thistechnology of a heat transfer apparatus is known as per U.S. Pat. No.4,515,209, for example. The first LHP systems were dedicated toterrestrial applications. Later, a capillary link (secondary wick)between the evaporator and the compensation chamber was introduced toprovide liquid supply from the compensation chamber to the evaporatorprimary wick in zero gravity (0-g) conditions.

The development and testing of a LHP with two identical evaporators wasfirst performed by the Institute of Thermal Physics (Russian Academy ofSciences) in the mid-80's. Further developments into a multi-evaporatorLHP system, as shown for example in USSR Patent 1395927, were carriedout using a LHP with two evaporators and two condensers. Thetwo-evaporator LHPs can efficiently operate at symmetrical andnon-symmetrical heat load distributions between the evaporators, and atdifferent temperatures of the condenser(s) cooling. However, shuttingdown the active cooling of one condenser would result in an abruptdecrease in the maximum transport capability of the device.

Every evaporator in the typical LHP system has its own compensationchamber, which can be directly connected to the compensation chambers ofother evaporators or can have no direct connection with the compensationchambers of other evaporators in the system. In these devices,evaporators are rigidly connected with each other and are at arelatively close distance from each other.

Despite evident advantages of LHP systems having multiple evaporatorsdesigned to operate over a wide temperature range, there exists alimitation on the number of evaporators that can be reasonably used, aseach evaporator comprises a compensation chamber. As the minimumoperating temperature decreases, the compensation chamber volumeincreases rapidly when the number of evaporators increases. This leadsto a limitation on the number of evaporators that can be used in thesesystems.

Besides, certain problems can also exist with the temperature control inmulti-evaporator LHP systems: the key components for the LHP temperaturecontrol are the compensation chambers. In a two-evaporator installation,the LHP can operate at the desired temperature in most of the cases, asthe LHP responds very well to rapid changes of heat load, sinktemperature and set point temperature. However, only one of thecompensation chambers has a vapor-liquid two-phase condition during theoperation regardless of how many are under temperature control.

The heat, which passed by thermal conduction through the capillary pumpwall into the central part of the evaporator, in the direction oppositeto the fluid circulation direction, is usually called parasitic heatleak. Test results showed that when one of the evaporators has a verylow heat load, a sudden vapour generation on the inner surface of thecapillary pump was observed, stridently increasing the parasitic heatleak to the compensation chamber which results in a higher operationaltemperature of the loop. This causes a hysteresis control problem forthe loop that is hard to predict or prevent. Also, it was found thatsituations when the liquid distributes itself among the compensationchambers (trying to occupy the lowest pressure spots) can lead tounstable operation of the system. Furthermore, a problem ofcontrollability for multi-evaporator LHP systems arises when the amountof evaporators and compensation chambers increases.

Therefore, it is possible to conclude that an expandability limitationis the main problem in multi-evaporator LHP systems, as shown in USSRPatent 1395927, such that two evaporators are used or only threeevaporators maximum for narrow temperature ranges. A secondary problempresented by these systems too is poor controllability.

Another type of capillary driven loop is CPL, as for example indocuments U.S. Pat. No. 6,626,231 and U.S. Pat. No. 7,118,076, typicallycomprising one or more evaporators, one or more condensers, transportlines, one remote compensation chamber and a sub cooler. Location ofcompensation chamber is the main distinguishing feature between CPL andLHP designs. LHP compensation chamber(s) is always directly attached toevaporator(s) but CPL has one remote compensation chamber (also known asliquid reservoir), separated from evaporator(s) by small diameter (2-5mm) connecting tube(s). In CPL, liquid from the condenser and from theremote compensation chamber flows through the sub cooler before reachingthe evaporators. The CPL comprising a remote reservoir loses ability forself-start up without special preconditioning. Besides, for any CPL, thetolerance for vapour parasitic heat leak is a significant problem ofreliable operability of the system. The growing of a vapour bubble onthe inner surface of the capillary pump leads to the pump dry out and,finally, to the failure of CPL operation. In case of LHP, the bubbleusually migrates into the compensation chamber (as soon as it is closelyattached to the evaporator) and condenses in sub cooled liquid which isalways presented in the LHP compensation chamber.

Continued improvements have been made to the CPLs in the last decades.The two-port evaporator (one liquid inlet and one vapour exit) initiallyused in CPLs generally experienced dry-out due to the appearance ofvapour in the liquid core during start-up and transient regimes. Toprevent vapour from blocking liquid return to the wick structure, athree-port capillary evaporator was introduced in the system connectingthe remote reservoir line to the liquid core of the evaporator. Thisconfiguration allows vapour to expand along the evaporator core and tomigrate into the remote reservoir, instead of accumulating in theevaporator core and interfering with liquid returning from thecondenser. Initially, three-port capillary pumps were used as starterpumps, and then like the main functional evaporator design. To preventvapour from depleted evaporators to flow upstream and to block liquidreturn to operating evaporators, a capillary device, known as acapillary isolator, was introduced, located upstream of the evaporatorinlet. Back pressure regulators were also installed in many multipleevaporator CPLs to assist start up. These capillary devices, located inthe vapour transport line, redirect vapour initially generated at oneevaporator to other inoperative (without heat load) evaporators. Thisaction forces liquid from the vapour lines and improves the chances fora successful start up for all evaporators in the system: it is alsohelping to promote heat load sharing among evaporators, for instance,when an inoperative evaporator acts as a condenser.

Furthermore, another problem in the known CPL systems is the formationof non-condensable gases in the loop, which can lead to evaporatorfailure if the non-condensable bubbles reach the evaporator coreblocking the liquid return to the evaporators of the CPLs. Sinceevolution of non-condensable gases over the CPLs lifetime is practicallyunavoidable, CPLs should be designed to be tolerant to non-condensablegases in one way or another. One of the possible solutions is toimplement special traps to collect the bubbles. The traps are usuallyused for systems with parallel condensers and are placed at thecondenser exit where they can also serve as capillary flow regulators(if the trap utilizes a capillary structure to separate gas fromliquid). The capillary structure helps to prevent vapour from leavingthe condenser. If one of the condensers becomes fully utilized, thenthis trap can serve to redirect the flow to the other condenser(s).

The following conclusions summarize the issues related to CPLreliability:

-   -   CPL design should never allow bubbles to form in the liquid side        of the loop: a bubble trap should then be provided at the outlet        of the sub cooler to prevent convection of non-condensable gases        or/and vapour bubbles to the evaporators;    -   CPL requires a start up evaporator to clear the vapour channels        in the main evaporators before heat is applied to them;    -   reducing the diameter of the CPL evaporator elements leads to        many unexpected difficulties: the design with thinner capillary        pump walls leads to higher probability of vapour bubble        formation inside of the liquid core of the evaporator and as        consequence to failure of CPL operation;    -   it is known in the state of the art, that in order to improve        vapour parasitic heat leak tolerance of evaporators, it is        preferable to connect these evaporators in series; in this case        the first evaporator in series creates sweeping flow for the        previous evaporators.

Another solution is to have several parallel evaporators connected tothe same compensation chamber, located at the evaporating part of theloop, and including special long capillary links between the evaporatorsand the compensation chamber. This system is known as Free Location LHP,as shown for example in documents U.S. Pat. No. 5,944,092 or RussianPatent 2120592. This system was successfully tested on the ground with afavourable gravitational bias of the evaporators relative to thecompensation chamber, making it easy for the capillary links todistribute the fluid to each evaporator. Orientation constraint ingravity field is due to limits imposed by the capillary link. Thecapillary link connecting the evaporators to the compensation chamberlimits the separation distance between the evaporators and thecompensation chamber. This limitation is similar to the existing inconventional heat pipes. Other significant limitations of this designare complexity and integration difficulties which lead to problems ofsystem expandability, scalability and part standardization. Allevaporators have to be below or in the same plane with respect to theplane of the compensation chamber. Since the tube connecting eachevaporator to the compensation chamber contains a capillary link inside,the tube internal diameter is typically greater than 4 mm, since it ispractically impossible to allocate a capillary structure in smallerdiameter tubing. Large diameter connecting tubing leads to inflexiblesystem and high requirements for tolerances for integration purposes. Inusual design of a LHP evaporator with a bayonet tube, a capillary link(secondary wick) supplies the primary capillary pump with liquidpractically only in transient regimes. However, in this design, thecapillary link supplies all amount of liquid that is needed for theevaporator, which leads to significant limitations for rates of changeof heat source power or/and heat sink temperature. Other disadvantage ofsuch approach is the low thermal conductance of evaporators due to thepermanent presence of vapour phase in the evaporator core.

An attempt to overcome some of these significant drawbacks led to a socalled multi-free LHP CPL known for example per U.S. Pat. No. 5,944,092,where functional evaporators do not have a capillary link to thecompensation chamber, only to the liquid line. Limitations of thisdesign are similar to those of ordinary CPLs with starter pumps.Capillary evaporators linked to the liquid line cannot provide areliable vapour tolerance and, therefore, this design presents thedrawback of the necessity of an additional special evaporator withdedicated power source to provide the loop circulation.

Further designs were made developing the so called multi-evaporatorhybrid LHP, as known for example in documents U.S. Pat. No. 7,661,464,U.S. Pat. No. 6,889,754, U.S. Pat. No. 7,004,240, U.S. Pat. No.8,047,268, U.S. Pat. No. 7,549,461, U.S. Pat. No. 8,109,325, U.S. Pat.No. 8,066,055, or U.S. Pat. No. 7,251,889, suggesting that a linkbetween evaporators and compensation chamber could itself be a loop andincorporated this idea in a so called advanced CPL, as an attempt toincorporate both the advantages of a robust LHP and the architecturalflexibility of a CPL. This system comprises two relatively independentlyoperated loops, a main loop and an auxiliary loop. The main loop isbasically a traditional CPL with same as for CPL configuration andoperational principles, whose function is to transport the waste heatand reject it to a heat sink via the primary condenser. The auxiliaryloop is used to remove vapour bubbles from the core of the CPLevaporators and move them to the compensation chamber. The auxiliaryloop contains only one LHP-type evaporator with the attached largecompensation chamber. The chamber is only one and it is common for allevaporators: the CPL evaporators in the main loop and the LHP evaporatorin the auxiliary loop. In addition, the auxiliary loop is also used toease the start-up process. In this manner, the auxiliary loopfunctionally replaces the secondary wick of a conventional LHP. Thefeasibility of this design was however only achieved when theevaporators were connected in series. This means that liquidconsequently goes through the evaporators: flow leaving the firstevaporator enters the second one, etc.

Initially, the multi-evaporator hybrid LHP included three evaporators,one of which was a standard LHP evaporator directly attached to thecommon system's compensation chamber, and two traditional three-port CPLevaporators. Tests indicated that the system was not very reliableduring power cycling. The sensitivity to power cycle was attributed tothe expansion of vapour bubbles in the evaporator core. Heat conductionthrough the wall of the evaporator capillary pump made it relativelyeasy to nucleate vapour in the evaporator core. In case of steady stateoperation, these bubbles were swept from the core of functionalevaporators by forward flow of the liquid to the capillary pump.However, as the functional evaporators input power decreased, liquidmovement forced by capillary action on the auxiliary evaporator was notenough to efficiently remove all vapour bubbles from the evaporator coreto prevent vapour blockage of the capillary pump (dry-out) after suddenincrease of the evaporator power. On the other hand, sudden powerreduction leads to temporary fluid flow break in the condenser until newstable temperature/pressure equilibrium was established in the system.This flow break therefore required a net flow mass displacement from theevaporator and the compensation chamber to the condenser. As a result,nominal forward direction flow was disrupted. During this reversal flow,vapour bubbles could then accumulate or even expand in the evaporatorcapillary pump core, therefore causing evaporator dry-out and failure ofthe system.

To improve vapour tolerance, the internal design of the evaporators wasmodified to include a special phase separation wick, designed to providebetter control of the two phases vapour/liquid distribution in the coreof the pumps. The design modifications were intended to extend the phasecontrol provided by the secondary wick in the traditional LHP evaporatorto the CPL evaporators. Despite general successful results obtainedduring testing, the operation was verified in relatively limitedconditions: mostly in horizontal orientation, evaporators were locatedclose to each other, and therefore with similar hydraulic resistance oflines. Therefore, such configuration was not representative of theconditions of potential spacecraft thermal control application whenevaporators and remote reservoir are spatially separated, and the rateof evaporators response on variations of the input power and heat sinkconditions depend on the length of the lines connecting these elements.Therefore, the ability for temperature control was not properlyverified.

Also known in the art are hybrid cooling loop technology, as those shownfor example in documents U.S. Pat. No. 6,990,816 and U.S. Pat. No.6,948,556, which combine the active liquid pumping with the passivecapillary liquid management in the wick structure of the evaporator andits liquid/vapour separation. The hybrid cooling loop consists of anevaporator, a condenser, a liquid compensation chamber and a pump as thesimplest design. Because of the active amplificatory pumping system, thehybrid loop system could manage different multiple evaporator designs.Despite certain advantages, the necessity of the supplementary loopcirculation means can be considered as a drawback because of the activecharacter of critical design components which reduces the reliabilityand life time of the system.

Another known system developed is the so called advanced LHP which is aLHP with two evaporators: main (functional) and secondary (auxiliary)evaporators, as per document U.S. Pat. No. 6,810,946 B2, for example,incorporating a secondary evaporator to the conventional LHP design. Thesecondary evaporator is located in a cold-biased environment to ensurethat its capillary pump is always primed. Electrical heaters areattached to this evaporator to provide the necessary thermal power forits functioning. With the secondary pump operating, it actively removesthe vapour that is accumulated in the compensation chamber by theparasitic heat leaks to the compensation chamber of the main evaporatorand to the liquid line. This design considers only a single mainevaporator LHP. The main drawback of this approach is the existence ofthe additional evaporator and its active character. In fact, thissolution is needed only for a LHP with not properly designed secondarypump.

Further, an evaporator with attached compensation chamber was proposedto use in a capillary driven loop, known for example per documents U.S.Pat. No. 7,061,446, U.S. Pat. No. 7,268,744 or U.S. Pat. No. 7,841,392.The undivided large capillary wick is used in the evaporator portion andin the compensation chamber. The wick has greater transverse size in thecompensation chamber than in the evaporator portion. There are no meansto guarantee vapour tolerance of the evaporators.

Thus, as a summary, it is possible to conclude that the main and themost critical element in a capillary driven loop is the evaporator. Thevapour and non-condensable gases intolerance, which can lead to totalfailure of the system in heat transfer, is the main problem in thedevelopment of capillary driven multi-evaporator two phase thermalcontrol systems. Various methods have been proposed and investigated tosolve the problem; however, the existing technical solutions stillcannot guarantee reliable and stable performance in different actualthermal conditions of spacecraft operation.

The present invention is therefore oriented towards these needs.

SUMMARY OF THE INVENTION

The present invention therefore provides a heat transfer and thermalcontrol system, in particular, a two-phase capillary driven LHP system.

An object of the invention is to provide a two-phase capillary drivenLHP system having reliable operation at a wide range of operationconditions, providing at the same time vapour parasitic heat leaktolerance means for the evaporator and design flexibility byimplementation of remote compensation chamber.

Another object of the present invention is to provide a two-phasecapillary driven LHP system that can be expanded, that is, that can varythe number of its evaporators and/or its condensers.

Other objects of the two-phase capillary driven LHP system of theinvention are the following:

-   -   scalability the size (both diameter and length) of the        evaporators can vary in a wide range and can be adjusted for any        particular application needed;    -   controllability: possibility to control the operating        temperature of the system by thermal control of the remote        compensation chamber;    -   capability of heat load sharing when the two-phase capillary        driven LHP system comprises multiple evaporators: power ranges        can be different for each evaporator, such that some evaporators        can have the maximum heat load while others have no power        application;    -   configuration flexibility: theoretically, an unlimited number of        evaporators/condensers can be used; the distance between        evaporators and compensation chamber can be up to several        meters; evaporators, condensers and remote compensation chamber        can be located in gravity field at various levels with elevation        difference up to 1-3 m taking into account only capillary        potential of evaporators secondary pumps;    -   functional flexibility there exists a wide range of heat input        powers for the entire system and for every evaporator;        resistance to rapid change of power inputs or/and condenser        temperatures occurs (related to the main objective of the        invention: vapour parasitic heat leak tolerance);    -   integration flexibility: small diameter (1-2 mm) tubing        connecting evaporators with remote compensation chamber allows        easy installation of the system on the satellite level; also,        flexible inserts such as tube coils or/and flexible hoses can be        used for better integration of the system;    -   evaporators standardization: possibility of using compensation        chambers attached to the evaporators, having standardized        dimensions without the need of effecting any re-qualification of        the evaporators for every configuration and size of the system;        this is especially important for the improvement of the        mechanical viability of the two-phase capillary driven LHP        system during vibration, as every evaporator of the system has        relatively small standardized individual compensation chamber        (simpler mechanical design as for the evaporators with the large        chambers) and can be mechanically designed and qualified        individually only one time.

These objectives are achieved with a two-phase capillary driven LHPsystem, effecting heat transfer and thermal control applications with atwo-phase fluid as a working media. The system of the inventioncomprises at least one evaporator, comprising a thermalstabilization-compensation chamber attached to it, at least onecondenser, liquid and vapour lines, and a single remote compensationchamber, the thermal stabilization-compensation chamber comprisingtwo-phase and hydro accumulator reservoirs. The remote compensationchamber is hydraulically connected with the two-phase and hydroaccumulator reservoirs of the thermal stabilization-compensationchamber. The evaporator comprises a primary capillary pump which servesto absorb heat from the equipment, which has to be cooled, and toprovide fluid heat continuous circulation between the evaporator, whichis connected to the heat source, and the condenser, which is connectedto the heat sink. A secondary capillary pump is located inside theprimary wick and inside of the thermal stabilization-compensationchamber and serves to supply the primary wick with liquid, and toprovide fluid/heat intermittent circulation in transient regimes ofoperation of the system, between the inner part of the primary wick andthe thermally controlled remote compensation chamber. In steady stateregimes of operation of the system, the thermalstabilization-compensation chamber serves to remove internal heat leakthrough a primary capillary pump by convection and condensation on theheat exchanger surface, which separates the two-phase and hydroaccumulator reservoirs in the thermal stabilization-compensationchamber.

Other features and advantages of the present invention will be disclosedin the following detailed description of illustrative embodiments of itsobject in relation to the attached figures.

DESCRIPTION OF THE DRAWINGS

The features, objects and advantages of the invention will becomeapparent by reading this description in conjunction with theaccompanying drawings, in which:

FIGS. 1 a, 1 b and 1 c show schematic views of the LHP device of theinvention having a remote compensation chamber and two evaporators.

FIG. 2 shows a general view of the LHP device of the invention havingmultiple evaporators (4 units) and multiple condensers (2 units).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a LHP device 1 comprising evaporator 2containing a stabilization-compensation chamber 10, a combination of aprimary capillary pump 30 and a secondary capillary pump 40, togetherwith the corresponding plumbing components of the LHP device 1. Theprimary capillary pump 30 serves for pumping fluid in the LHP device 1,the evaporation of which absorbs heat from the system that has to becooled. The secondary capillary pump 40 serves for supplying liquid tothe primary capillary pump 30 and, together with thestabilization-compensation chamber 10 and the remote compensationchamber 20, for providing means to remove the vapour that is formed byinternal parasitic heat leak of the at least one evaporator 2.

The present invention relates to a LHP device 1, which can be of thetype single evaporator-condenser or multiple evaporators (and forcondensers) embodiments, as shown in FIGS. 1 a, 1 b, 1 c. The LHP device1 of the invention comprises the following components:

-   -   at least one evaporator 2: the evaporator 2 comprises        stabilization-compensation chamber 10, a combination of a        primary capillary pump 30 and a secondary capillary pump 40. The        primary capillary pump 30 serves for pumping fluid in the LHP        device 1, the evaporation of which absorbs heat from the        equipment that has to be cooled. The secondary capillary pump 40        serves for supplying liquid to the primary capillary pump 30        and, together with the stabilization-compensation chamber 10 and        the remote compensation chamber 20, for providing means to        remove the vapour that is formed by internal parasitic heat leak        18 of the at least one evaporator 2;    -   one remote compensation chamber 20 in two-phase condition for        temperature control functions and for managing changes of liquid        phase volume together with excessive vapour parasitic heat leaks        in the LHP transient regimes of operation, providing compact        standardized design of evaporator 2 and expandability in the        embodiment having multiple evaporators 2; there is no need of        the stabilization-compensation chambers 10 having large volumes,        depending on total volume of the LHP device 1, as they can have        minimal unified volumes, enough to manage and ensure        vapour/non-condensable gases tolerance in steady state regimes;    -   at least one condenser 27;    -   vapour line 28 and liquid line 24.

FIGS. 1 a, 1 b and 1 c show different schemes of an embodiment of theinvention, showing a LHP device 1 having an arrangement of a remotecompensation chamber 20 and evaporators 2, such that:

-   -   FIG. 1 a shows the remote compensation chamber 20 being        connected to a two-phase reservoir 5 of the        stabilization-compensation chamber 10 by a two-phase line 12,        the liquid accumulator reservoir 6 of the        stabilization-compensation chamber 10 being connected to the        remote compensation chamber 20 by a liquid line 13. The returned        liquid from the condenser is always passed through remote        compensation chamber 20 before to arrive to evaporators 2.    -   FIG. 1 b shows the remote compensation chamber 20 being        connected to the two-phase reservoir 5 of the        stabilization-compensation chamber 10 by a two-phase line 12,        the liquid line 13 being directly connected to the liquid line        24 returning liquid to a bayonet tube 7 at the entrance of the        evaporator 2 from the condenser 27. Via lines 13 and 24 the        remote compensation chamber 20 has a hydraulic link with the        liquid accumulator reservoir 6 of the stabilization-compensation        chamber 10.    -   FIG. 1 c shows the remote compensation chamber 20 being        connected to the two-phase reservoir 5 of the        stabilization-compensation chamber 10 by a two-phase line 12,        the stabilization-compensation chamber 10 also comprising a        liquid accumulator reservoir 6 directly connected to the remote        compensation chamber 20 by a liquid return line 13.

The three presented cases illustrate different variants of the remotecompensation chamber 20 designs and different ways of layout in the LHPdevice 1. The two-phase port of the remote compensation chamber 20 isalways connected via line 12 with the stabilization-compensation chamber10. However liquid port(s) of the remote compensation chamber 20 can beconnected to stabilization-compensation chamber 10 in 3 differentmanners: directly, through liquid line 24 in series and in parallel. Twois the minimum amount of fluid ports (one is for two-phase line 12 andsecond for liquid return line 13. The maximum quantity of fluid portsfor remote compensation chamber 20 can be calculated by multiplying thenumber of evaporators by two and adding the number of condensers: inthis case every evaporator has two individual lines 12 and 13 joiningthe stabilization-compensation chamber 10 with remote compensationchamber 20 and the remote compensation chamber 20 has additional liquidlines 24 connected with the condenser. Different combinations betweenmaximum and minimum amount of ports are possible and it also providesflexibility in the system design.

The numerals shown in FIGS. 1 a-1 b-1 c and 2 represent the following:

-   -   1 LHP device    -   2 Evaporator    -   3 Vapour-liquid interface    -   4 Separator of low and high pressure sides of the primary        capillary pump 30    -   5 Two-phase reservoir linked to the internal core of the primary        capillary pump 30    -   6 Liquid accumulator reservoir    -   7 Bayonet tube, liquid transport line entrance from condenser 27    -   8 Liquid flow direction    -   9 Heat sink    -   10 Stabilization-compensation chamber    -   11 Vapour flow direction    -   12 Two phase line to the remote compensation chamber 20    -   13 Liquid return line of the remote compensation chamber 20    -   14 Fluid in liquid state    -   15 Heat exchanger (heat exchange surface)—separator of low        (liquid) and high (two-phase) pressure sides of the secondary        capillary pump 40    -   16 Vapour-removing channels inside the primary capillary pump 30        (evaporator core)    -   17 Heat input    -   18 Parasitic heat leak into the central core of the primary        capillary pump 30    -   19 Vapour-removing channels outside the primary capillary pump        30    -   20 Remote compensation chamber    -   22 Vapour bubbles    -   23 Liquid drops    -   24 Liquid transport line    -   25 Porous wick inside the remote compensation chamber 20    -   26 Liquid channel    -   27 Condenser    -   28 Vapour line    -   29 Fluid in vapour state    -   30 Primary capillary pump    -   31 Fluid in two-phase state    -   40 Secondary capillary pump

The evaporator 2 comprises a small stabilization-compensation chamber 10containing a secondary capillary pump 40, designed in such a way that itefficiently manages the vapour flow due to the parasitic heat leak 18into the central core of the primary capillary pump 30.

The evaporator 2 design comprises a primary capillary pump 30 withexternal vapour-removing channels 19 outside the primary capillary pump30, a secondary capillary pump 40 and a stabilization-compensationchamber 10 which comprises two chambers, a two-phase reservoir 5 and aliquid accumulator reservoir 6. The primary capillary pump 30 alsocomprises internal vapour-removing channels 16 in the evaporator 2 core,to remove the vapour that forms due to the heat leak through the primarycapillary pump 30. These vapour-removing channels 16 are connected withthe small two-phase reservoir 5 close to the vapour-removing channels 16outlets. This two-phase reservoir 5 comprises a heat exchanger 15 (heatexchange surface) between the two-phase reservoir 5 and the liquidaccumulator reservoir 6 of the stabilization-compensation chamber 10.The liquid accumulator reservoir 6 and the two-phase reservoir 5 withthe heat exchange surface 15 can be called as stabilization-compensationchamber 10. The secondary capillary pump 40 is located inside of theprimary capillary pump 30 and the stabilization-compensation chamber 10.A porous wick 25 is installed inside of the remote compensation chamber20 to manage fluid distribution in micro gravity conditions. The porouswick 25 prevents also vapour or non condensable gas bubbles penetrationto the liquid line 13 as well as to the liquid accumulator reservoir 6.

Working fluid exists in three states inside of the LHP device 1 of theinvention: vapour 29, liquid 14 and two-phase 31 states.

When heat 17 is supplied to the evaporator 2 by the heat releasingequipment or heat source, the heat evaporates working liquid. Vapourgoes from the evaporator 2 to the condenser 27 through the vapourtransport line 28, where it is condensed. After that, the working liquidreturns to the stabilization-compensation chamber 10 and to theevaporator 2 through the liquid transport line 24, to be againevaporated in the primary capillary pump 30 of the evaporator 2. Unlikeordinary LHP systems, the proposed LHP device 1 of the invention iscontrolled by the remote compensation chamber 20, as two-phases arealways present in this chamber.

The link of the secondary capillary pump 40 and thestabilization-compensation chamber 10 provides the following functions:

-   -   redistributes and supplies liquid from the bayonet tube 7 and        internal liquid channel 26, supplying it to the primary        capillary pump 30 (mainly in steady state regimes);    -   transports liquid from the remote compensation chamber 20        through the liquid accumulator reservoir 6, supplying it to the        primary capillary pump 30 (mainly in transient regimes);    -   together with the stabilization-compensation chamber 10 and        remote compensation chamber 20, it provides vapour parasitic        heat leak tolerance passive means individually for every        evaporator 2 (of multi-evaporator design).

The LHP device 1 can contain several evaporators 2 and severalcondensers 27 (FIGS. 1, 2). It is provided the opportunity that theevaporators 2 can collect the power from different heat sources, whichcould be located far one from the others thanks to theflexibility/adaptability provided by the LHP device 1 concept:

Various embodiments of present invention regarding the power rejectionare possible. Even for single-evaporator LHP device 1, severalcondensers 27 can be placed in different locations to take advantage ofthe most favourable conditions of sink depending on the position alongthe orbit (for space applications of the LHP device 1), for example, twoparallel condensers 27 can be located in opposite faces (FIG. 2).

Several means of vapour tolerance management are designed forcompensating primary heat leak penetrating through the primary capillarypump 30 to the evaporator 2 core and for compensating secondaryparasitic heat leak penetrating through the secondary capillary pump 40(which is significantly, in order of magnitude lower than the primaryparasitic heat leak):

-   -   the heat exchanger 15 in the stabilization-compensation chamber        10 provides the possibility to cool and condense vapour        generated by the main (primary) parasitic heat leak 18; the cold        sub-cooled liquid in the liquid accumulator reservoir 6 cools        and condenses vapour bubbles 22 when liquid exists in the        two-phase reservoir 5 or condenses vapour with forming drops of        liquid 23 on the heat exchange surface 15, the heat exchanger 15        being designed having a surface area calculated to condensate        vapour corresponding to 10-15% of the evaporator input heat load        17 (maximum possible values of the heat leak), so that the        two-phase line 12 is usually filled with liquid, which is the        nominal regime of the LHP operation in steady state conditions,        the heat exchanger 15 being the main means of vapour parasitic        heat leak tolerance;    -   a self-induced “core sweepage” mechanism to ensure compensation        of the parasitic heat leak during transient regimes, such that        the secondary capillary pump 40 guarantees the removal of vapour        from the evaporator core 16 to the remote compensation chamber        20 and the liquid return 13 to the stabilization-compensation        chamber 10, which is especially important during transient        operation regimes (change of input heat 17 or/and condenser        temperature) with elevated heat leak;    -   design of the stabilization-compensation chamber 10 as a cold        liquid accumulator, providing effective compensation of        secondary parasitic heat leak through secondary capillary pump        40.

Thus, the main vapour/non-condensable gases tolerance means are locatedin maximum proximity to the evaporators 2. Moreover, not only the liquidflowing from the condenser 27 reaches the evaporator 2, but also liquidstorage in the liquid accumulator reservoir 6 can be supplied to theevaporator 2 when required (mainly in transient regimes), providingadditional reliability for the system. Besides, several additionalredundant means can be considered: auxiliary LHP and/or thermalelectrical cooler, for example.

Vapour generated by internal heat leak 18 in the evaporator core movingto the two-phase reservoir 5 is condensed by the heat exchanger 15(nominal case operation). Therefore the two-phase line 12 connecting thetwo-phase reservoir 5 and the remote compensation chamber 20 is usuallyfilled with liquid.

During the most unfavourable transient regimes part of vapour 11, whichcould not be condensed completely on the heat exchange surface 15 in thestabilization-compensation chamber 10, can go to the remote compensationchamber 20 to condensate there. The rest of heat leak (secondary leakpenetrating to the liquid channel through the secondary capillary pump40 will be compensated with condensation in the liquid accumulatorreservoir 6 in the stabilization-compensation chamber 10 by sub-cooledliquid.

Presence of the remote compensation chamber 20 gives the opportunity tomanage non-condensable gas inside of the LHP. In typical LHP,non-condensable gas is located in the compensation chamber 10 inproximity of the evaporator 2 and can penetrate to the evaporator core16 and thus influence more significantly the evaporator 2 and therefore,the LHP operation. In the proposed design according to the invention,non-condensable gas will move to the remote compensation chamber 20 andit will accumulate non-condensable gas preventing negative impact on LHPoperation.

Such scheme guarantees vapour/non-condensable gases tolerance of the LHPdevice 1 and system reliability (especially in transient regimes)individually, passively and automatically for every evaporator 2 (ofmultiple evaporator option), without the necessity of having an activecontrol. This design is a simpler and more robust alternative to theactive external “forced pumping” designs of known technical solutions inthe prior art equipped with remote auxiliary capillary or mechanicallypumped loops for the entire system. The secondary capillary pump 40 isworking as a capillary pump of the secondary loop with remotecompensation chamber 20 as a condenser to absorb heat leak through theprimary capillary pump 30. Thus, the secondary capillary pump 40 hassimilar function as a remote auxiliary capillary or mechanically pumpedloop in known designs.

A remote compensation chamber 20 (common for all evaporators 2 ofmultiple evaporator option) included in the proposed design serves toaccumulate liquid and to compensate the liquid volume changes during theLHP device 1 operation. This large reservoir helps to avoid theobligation of designing a large volume compensation chamber for theindividual evaporators in the multiple evaporator option (in ordinaryLHPs with multiple evaporators their volumes depend strongly on thetotal number of evaporators 2 in the system). Therefore, thisconfiguration allows having a scalable design which can be fitted easierto the required number of evaporators 2 and the specific requirements ofeach application, because evaporators 2 will have same designindependently on the design and volume of the lines, condensers 27,total number of evaporators 2, etc. Only the volume of the remotecompensation chamber 20 has to be adjusted for specific requirements.

The design and location of the remote compensation chamber 20 can beselected depending on the functional purposes and the geometricalconstraints. However, it is recommendable to control the temperature ofthe remote compensation chamber 20. For these purposes, several optionscan be considered and the best solution can be selected depending oneach application requirements:

-   -   to have an active control by using a heater or thermal        electrical cooler, to control the temperature and facilitate the        priming of the loop prior to the start up;    -   to have a heat link with the environment to maintain its        temperature in a certain range.

The LHP device 1 of the invention can comprise several optionaladditional elements, such as:

-   -   a subcooler located between the condenser 27 and the outlet of        the liquid line 24;    -   a capillary blocker can be installed at the outlet of parallel        condensers 27 for a better vapour distribution between them;    -   additional capillary blockers could be also introduced at the        outlets of the liquid lines 24 of multiple evaporators 2 to        prevent the liquid line 24 and the evaporators 2 to experience        pressure drops.

The LHP device 1 of the invention may further comprise externalauxiliary means such as cold bias links or thermal electric coolers forsubcooling the liquid inside the liquid accumulator reservoirs 6 in thestabilization-compensation chambers 10.

There also exist other auxiliary elements to provide subcooling to theliquid accumulator reservoirs 6 in the stabilization-compensationchamber 10: this cooling is settled mainly as additional means to removethe back conduction of the evaporators 2 and parasitic heat leak to theliquid line 24. Thus, several options are considered:

-   -   cold bias links;    -   thermal electric coolers located on the        stabilization-compensation chamber 10.

All the above-mentioned options should be carefully evaluated for everyparticular case depending on the operational conditions desired for theLHP device 1.

Although the present invention has been fully described in connectionwith preferred embodiments, it is evident that modifications may beintroduced within the scope thereof, not considering this as limited bythese embodiments, but by the contents of the following claims.

1. Loop heat pipe apparatus for heat transfer and thermal control, usinga two-phase fluid as a working media and comprising: at least oneevaporator to be connected with a heat source and comprising a thermalstabilization-compensation chamber attached to said at least oneevaporator and a secondary capillary pump located inside said thermalstabilization-compensation chamber, at least one condenser to beconnected with a heat sink, liquid lines and vapour lines connectingsaid at least one evaporator and said at least one condenser, and aremote compensation chamber, wherein said thermalstabilization-compensation chamber comprises a two-phase reservoir and aliquid accumulator reservoir separated by a heat exchange surface, suchthat said remote compensation chamber is hydraulically connected withsaid two-phase reservoir and said liquid accumulator reservoir.
 2. Loopheat pipe apparatus for heat transfer and thermal control, according toclaim 1, further comprising a primary capillary pump comprising outervapour channels to collect and remove heat from a cooled device andinner vapour channels to collect and remove vapour produced by parasiticheat leak penetrating through said primary capillary pump.
 3. Loop heatpipe apparatus for heat transfer and thermal control, according to claim2, wherein said inner vapour channels are linked with said two-phasereservoir of said stabilization-compensation chamber where moved vapourgenerated due to parasitic heat leak is condensed on a dedicated heatexchange surface.
 4. Loop heat pipe apparatus for heat transfer andthermal control, according to claim 1, wherein said secondary capillarypump contains an inner liquid channel with a bayonet tube for liquidreturned from said condenser and said remote compensation chamber. 5.Loop heat pipe apparatus for heat transfer and thermal control,according to claim 1, wherein said remote compensation chamber has aninternal capillary structure which separates a liquid return line fromentire volume of said remote compensation chamber to prevent vapourflow/bubbles penetrating into said liquid return line and into saidliquid accumulator reservoir of said stabilization-compensation chamber.6. Loop heat pipe apparatus for heat transfer and thermal control,according to claim 5, wherein said remote compensation chamber isconnected with said two-phase reservoir of saidstabilization-compensation chamber by a two-phase line and with saidliquid accumulator reservoir of said stabilization-compensation chamberdirectly by said liquid return line.
 7. Loop heat pipe apparatus forheat transfer and thermal control, according to claim 5, wherein saidremote compensation chamber is connected with said two-phase reservoirof said stabilization-compensation chamber by a two-phase line and withsaid liquid accumulator reservoir of said stabilization-compensationchamber by said liquid return line and a liquid transport line.
 8. Loopheat pipe apparatus for heat transfer and thermal control, according toclaim 5, wherein said remote compensation chamber is connected with saidtwo-phase reservoir of said stabilization-compensation chamber by atwo-phase line and with said liquid accumulator reservoir of saidstabilization-compensation chamber by said liquid return line which hastwo functions, namely to transport liquid to a bayonet entrance of saidevaporator from said condenser via a liquid transport line, and toreturn liquid from said remote compensation chamber.
 9. Loop heat pipeapparatus for heat transfer and thermal control, according to claim 1,further comprising several evaporators.
 10. Loop heat pipe apparatus forheat transfer and thermal control, according to claim 1, furthercomprising several condensers.
 11. Loop heat pipe apparatus for heattransfer and thermal control, according to claim 9, comprising acapillary blocker in a liquid transport line in a liquid inlet of everyevaporator.
 12. Loop heat pipe apparatus for heat transfer and thermalcontrol, according to claim 10, comprising a capillary blocker in aliquid outlet of every condenser.
 13. Loop heat pipe apparatus for heattransfer and thermal control, according to claim 1, further comprisingexternal auxiliary means for subcooling of liquid inside of liquidaccumulator reservoirs of stabilization-compensation chambers.
 14. Loopheat pipe apparatus for heat transfer and thermal control, according toclaim 13, wherein said external auxiliary means for subcooling of liquidinside of liquid accumulator reservoirs of stabilization-compensationchambers are cold bias links or thermal electric coolers.